Another kind of Approximation, called best co-approximation, was introduced by Franchettei and Furi in 1972 [1]. Some results on best co-approximation theory in metric and linear normed spaces have been obtained by P.L. Papini, I. Singer, T.D. Narang, and others (see [1], [4], [5], [6]). Cone metric spaces was introduced by Huang and Zhang in 2007 [2]; where the set of real numbers is replaced by an ordered Banach space. They introduced the basic definition and discuss some properties of convergence of sequences in cone metric spaces. Recently, Sh. Rezapour has a research work on best approximation in cone metric spaces [7]. In this paper we want to investigate the concept of best co-approximation in cone metric spaces.